Fossils

Fossils are physical evidence of preexisting organisms, either plant or animal. The most common
and obvious fossils are the preserved skeletal remains of animals. Other fossils, which are also
evidence of past organisms, include leaf impressions, tracks and trails, burrows, droppings, and
root casts. Microfossils are the microscopic skeletons of previously existing plants or animals,
and their examination requires an optical or an electron microscope for close study. A very small
fraction of the organisms that have lived on the Earth is found in the fossil record: Many did
not possess skeletons or other hard parts that could be preserved; many did not survive the process
of fossilization, wherein skeletons and tissues are replaced by minerals; and many were subsequently
destroyed either by chemical or physical processes such as recrystallization, metamorphism, or erosion.

Fossils of any kind are useful in "reading the rock record," meaning they help us decipher the history
of the earth. They can help us determine the geologic age and environment (the paleoenvironment) in
which they were deposited. Finally, and if the fossil record is complete enough, their study can help
us better understand the evolution (or progression) of life through geologic time.

Our understanding of the meaning of fossils is based on the accumulated knowledge from previous
generations of investigators who carefully recorded the identity and distribution of fossils from
numerous geologic exposures or samples from wells and recorded their findings in the scientific
literature. When the regional or global distribution of fossils through geologic time is taken into
consideration, we can gain important insights into such phenomena as continental drift, community
migration, and climatic (paleoclimatic) reconstruction.